1. Field of the Invention
Generally, the present invention relates to the field of telecommunications devices for the deaf (TDDs). More particularly, the invention relates to service negotiation within a telecommunication system that allows a low activity code based signal to be transmitted by the system. The system may include wireless links.
2. Description of the Related Art Many deaf or hearing-impaired people use communication terminals specifically constructed and designed to enable them to communicate over standard telephone lines. Such devices, referred to as telecommunication devices for the deaf (TDDs) or Text Telephone Yokes (TTYs), are collectively referred to as TTDs in this application. Typically, TTDs include a keyboard and a display connected to a telephone via a modem (modulator/demodulator). The modem is built into the TDD and is either directly connected to a telephone line or coupled by an acoustic coupler to a normal telephone handset. TDDs are capable of transmitting information over telephone lines by means of coded tones to other TDDs connected at opposite ends of the telephone line through another modem. These tones are referred to as low activity communications because the frequency and amplitude envelopes remain relatively constant.
The code and protocol that is in widespread conventional use for TDD communications is an idiosyncratic one. The code set, known as Baudot, and the communication protocol (TDD protocol) evolved historically at a time when many telecommunication devices for the deaf were based on mechanical or electromechanical devices rather than electronic devices. Accordingly, the TDD protocol was constructed for a set of constraints that no longer are relevant to present day devices. Those constraints work to create a code protocol and a telecommunication network of users and devices operating under that protocol, that is somewhat antiquated.
Traditionally, TDD communications are conducted at 50 Baud (45.5 Baud in some countries), representing a transfer of 6 characters per sec. Other protocols now available for TDD communications incorporate higher Baud rates, such as the ASCII (American Standard Code Information Interchange) and enhanced Baudot protocols. Regardless, a normal TDD communication character set consists of characters that are 5 bits long. These characters are analogous to a letter in an alphabet where each letter represents a word or idea. A character is grouped with some information bits prior to transfer where each group of bits to be transferred has a duration or unit interval equal to 20 milliseconds. For example, under conventional TDD protocol, a group of bits to be transferred comprises 8 bits: a start bit (one source or zero bit), five bits representing the character, and at least one and 1/2 bits marking the stop point of the transfer group.
Compared to modem telecommunication systems, TDD transmissions occur at a snail's pace. A bigger problem is that TDD signals are substantially constant. These slow paced, monotone signals can create havoc in digital telecommunication systems that transmit higher activity signals at very high rates, especially telecommunication systems that include wireless links. One example of such a telecommunication system is a code division multiple access (CDMA) system having a large number of wireless subscriber units. Each subscriber unit has a transceiver and communicates within the system through satellite repeaters or terrestrial stations referred to as cells. Each cell includes a physical plant called a base station. A cell covers a limited geographic area and routes calls carried over subscriber units to and from the telecommunication network via a mobile switching center. When a subscriber moves into the geographic area of a new cell, the routing of that subscriber's call may be eventually made through the new cell by a process called a "handoff."
A subscriber unit, generically referred to as a cell phone, transmits a signal that is received by a base station. The signal is relayed to a mobile switching center that routes the signal to a public switched telephone network (PSTN) including telephone lines or other subscriber units. Similarly, a signal may be transmitted from the PSTN to a subscriber unit via a base station and a mobile switching center.
The interface between the subscriber unit and the base station is referred to as the air interface. The telecommunications industry association (TIA) has provided a standard for CDMA call processing on the air interface entitled "IS-95 Mobile Station Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System." Addendum to IS-95 are provided as Telecommunications Service Bulletins (TSB). The standard IS95+TSB74 includes provisions for service negotiation on the air interface and is incorporated herein by reference.
Service negotiation is critical to successfully transmit any communication, especially a low activity TDD communication, over a digital telecommunication system. One problem with modern systems, including the one described above, is that a vocoder--a device used in the system to encode a voice or TDD analog signal into a digital signal, and to decode a digital signal into a voice or TDD analog signal--has difficulty in handling the substantially monotone signal and slow speed dictated by the TDD protocol. Further, no method or device currently exists for noticing a telecommunication system that a TTD communication is being received and to make adjustments as required to successfully transmit a low activity signal. In current systems, a low activity communication signal such as a TDD communication would probably be treated by the vocoder as background noise or signal interference and be disregarded.
What is needed is a method and apparatus that can be easily integrated into existing digital communication systems, that can sense when a low activity communication, such as a TDD communication, is received, and that can notice the system to automatically negotiate the service to be used by the communication system during transmission of the low activity communication.
The method and device should be compatible with CDMA modulation systems and the techniques used in telecommunication systems servicing large numbers of system users. A more robust discussion of CDMA systems and techniques in this type of multiple access communication systems may be found in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," assigned to the assignee of the present invention and incorporated by reference herein. Further, the method and device should also be compatible with other modulation systems and techniques used in other communication systems, such as time division multiple access (TDMA), frequency division multiple access (FDMA), and amplitude modulation (AM) schemes.